This invention relates to electronic ballasts for gas discharge lamps and, in particular, to an electronic ballast that separately operates two or more lamps under external control.
A gas discharge lamp, such as a fluorescent lamp, is a non-linear load to a power line, i.e. the current through the lamp is not directly proportional to the voltage across the lamp. Current through the lamp is zero until a minimum voltage is reached, then the lamp begins to conduct. In many gas discharge lamps, small filaments at each end of the lamp are made to glow and emit electrons to facilitate starting the lamp. Such lamps are referred to as rapid start or program start lamps. The filaments are typically coated with a material having a low work function, that is, a material that emits electrons profusely when heated, thereby aiding in ionizing the gases within the lamp and reducing the voltage required to start the lamp. Once the lamp conducts, the current will increase rapidly unless there is a ballast in series with the lamp to limit current.
An electronic ballast typically includes a rectifier for changing the alternating current (AC) from a power line to direct current (DC) and an inverter for changing the direct current to alternating current at high frequency, typically 40-65 kHz. Some ballasts include a boost circuit between the rectifier and the inverter. As used herein, a xe2x80x9cboostxe2x80x9d circuit is a circuit that increases the DC voltage, e.g. from approximately 170 volts (assuming a 120 volt input) to 300 volts or more, for operating a lamp and for providing power factor correction. xe2x80x9cPower factorxe2x80x9d is a figure of merit indicating whether or not a load in an AC circuit is equivalent to a pure resistance, i.e. indicating whether or not the voltage and current are in phase. It is preferred that the load be the equivalent of a pure resistance (a power factor equal to one).
It is known in the art to control an electronic ballast with a microprocessor. U.S. Pat. 5,680,015 (Bernitz et al.) discloses a ballast for a high intensity discharge lamp wherein a microprocessor controls a driven half-bridge inverter having a series resonant, direct coupled output. U.S. Pat. 5,925,990 (Crouse et al.) discloses controlling a ballast for gas discharge lamps and for monitoring the operation of the lamps.
Despite the technology contained in an electronic ballast, the ballast is only part of a larger system including lamps and fixtures. In many installations, room lighting is controlled by two switches. Typically, one switch operates one of three fluorescent lamps and the other switch operates the remaining two lamps. The intention is that the full light output is not always required and hence energy can be saved by having reduced light output during parts of the day or evening.
Frequently, the way to separately control lamps is by having two three-lamp fixtures operated by three two-lamp ballasts or else have one two-lamp ballast and one four-lamp ballast. Each ballast operates lamps in both fixtures. A frequent arrangement is to have the center lamps in each fixture powered by a two lamp ballast in one of the fixtures. The remaining lamps are either operated by one four-lamp ballast in one fixture or else by a two-lamp ballast in each fixture. These configurations are pre-assembled at the factory in the form of a master fixture, a satellite fixture and a xe2x80x9cwhipxe2x80x9d or connecting conduit that extends between two fixtures. The fixtures and whip are shipped together as components which must be assembled in the manner intended. Further, the operating voltage for each assembly has to be specified in advance.
Shipping these related pieces and assembling them in the field is commonly described as a nightmare. As one can imagine from the number of combinations of components, confusion and mistakes are likely and cost the manufacturers and the contractors large sums of money.
The above-identified, co-pending applications address the problem of separately operating lamps and the solutions work well for instant start lamps. For rapid start lamps, a problem arises in that the filaments should be heated before applying a starting voltage to the lamp. Switching from one lamp to the other two lamps in a three lamp fixture, one can instant start a rapid start lamp simply by applying a sufficiently high voltage but this is hard on the filaments and decreases the life expectancy of the lamp. With preheat, one could program a controller to delay until the lamps are ready to start. This delay would be perceptible and a user might think that the switch or the ballast is defective and shut off all the lamps before the two lamps have had a chance to start. Alternatively, one could switch immediately to the two lamps and plunge the area into darkness momentarily until the lamps ignited. Neither solution is acceptable.
It is well known that marketing is based upon perception, not reality. The perception of delay as a defect is largely an American phenomenon. In Europe, familiarity with rapid start lamps causes an instant start to be seen as a defect because it is recognized that instantly starting a rapid start lamp is not good. Thus, a ballast suitable for both American and European markets must meet contrary perceptions.
Heating the filaments of all the lamps to be ready for any combination of lamp settings is possible but not an efficient solution because the power consumed by the filaments decreases the efficiency of the system and undermines the whole purpose of being able to select less than all the lamps in a fixture.
Typically, lamps are pre-heated by coupling the filaments to a frequency sensitive circuit that couples more power to the filaments at high frequency than at a lower frequency where the lamps normally operate. In this way, power to the filaments reduces automatically after a lamp has ignited. In order to heat the filaments, all the inverters in a ballast having plural inverters would have to be operating, which further decreases the efficiency of the system and undermines the whole purpose being able to select less than all the lamps in a fixture. Alternatively, operating immediately at low frequency (high output voltage) would cause an instant start, which is also undesirable.
In view of the foregoing, it is therefore an object of the invention to provide an electronic ballast that efficiently and selectively controls a plurality of rapid start, gas discharge lamps.
Another object of the invention is to provide an electronic ballast for rapid start lamps that can instantly change state when on.
A further object of the invention is to provide a electronic ballast that includes a single converter and a plurality of inverters, wherein each inverter performs a programmed start of the lamps connected thereto.
Another object of the invention is to provide an electronic ballast for rapid start lamps, the ballast having a plurality of power inputs and a plurality of inverters, wherein the operation of the inverters is controlled by the power inputs.
A further object of the invention is to provide an electronic ballast for selectively controlling plural loads that is acceptable to both American and European markets.
Another object of the invention is to provide an electronic ballast for selectively controlling plural rapid start, gas discharge lamps by delaying starting to simulate a pre-heat period even though the filaments are already warm.
The foregoing objects are achieved by this invention in which gas discharge lamps having filaments at each end thereof are operated in groups according to the power applied to separate line inputs to the ballast. The filaments of the lamps in a first group are powered by a first inverter that provides lamp current to a second group. The filaments of the lamps in the second group are powered by a second inverter that provides lamp current to the first group of lamps. Thus, even if an inverter is turned off, the lamps powered by that inverter are in a pre-heated state, ready for immediate conduction. For markets adversely sensitive to instant starting, a delay is programmed into the controller for the ballast to simulate a warm-up period, thereby avoiding the appearance of an instant start. Power is coupled to the filaments by a network that is relatively insensitive to changes in frequency.